In any modern aircraft, a variety of sensors are used to alert the pilot and crew to virtually every aspect of the aircraft's operating condition. For example, sensors are used to determine the position of the pilot's controls, the position of the flight control surfaces, the position of the landing gear, the amount of fuel remaining, the aircraft's speed etc. Traditionally, a sensor comprised an electromechanical device that produced an analog signal indicative of the parameter being sensed. The analog signals were transmitted over a copper wire to an onboard aircraft computer where they were converted into a corresponding digital signal and analyzed.
With advances in fiber optic technology, many older analog sensing systems are being replaced with digital optical sensors. A digital sensor can provide an absolute reading of a measurand in the presence of system noise and transmission path instability that is far more precise than could be obtained with an analog sensor. A typical optical sensor includes a light source, a fiber optic cable that carries light from the light source and a code plate that is coupled to the object whose position is to be sensed. The code plate has a pattern of nonreflective (absorptive) and reflective segments on it. Light striking the code plate is reflected back to a decoder at certain wavelengths, which are analyzed to provide an indication of the position of the object. Three of the most important advantages of optical sensors over the prior art analog sensors are the fact that they are lightweight, don't require electrical power into the sensor, and that the fiber optic cables are not susceptible to electromagnetic interference.
In most aircraft systems that use digital sensors, the drawback is that the loss of one bit of information can be catastrophic. Therefore, there are typically redundant sensors for each critical function of the aircraft to be monitored. Thus if one sensor fails, the other sensor can be used. However, when two sensors are used, it is often difficult or impossible to tell which sensor is producing erroneous data and the data from both redundant sensors must be ignored. Therefore, most redundant sensor systems include at least three identical sensors in order to determine which sensor, if any, is faulty. This approach is both costly and increases the complexity and weight of the aircraft.
To reduce the complexity of redundant sensor systems, attempts have been made to design optical sensors that have built in fault detection so that the sensor itself can tell when the data being provided is erroneous. For example, attempts have been made to use Manchester encoding in the code plate so that every bit is encoded as a logical one to zero or zero to one transition. This technique has not proved satisfactory because twice the number of reflective segments must be encoded on the code plate, thereby requiring more spectral resolution and a decoding circuit with increased resolution. Alternatively, code plates have been designed to include parity bits, which can detect an odd number of errors. However, parity bits cannot detect the presence of an even number of errors in the code plate. Therefore, there is a need for an optical sensor system that is self monitored, can detect more than a single error and does not require additional complex encoding and decoding methods.